Handover is a significant study issue in a wireless communication system. In general, when a mobile station (MS) decides to perform a handover, it transmits a handover request message to a base station (BS) and the base station replies with a handover response message. Upon receipt of the handover response message, the mobile station finally selects a target base station and transmits a handover indication message to the target base station, notifying of the handover. In this manner, the mobile station performs the handover.
A handover procedure in a broadband wireless access (BWA) communication system, for example, will be described with reference to FIG. 1.
FIG. 1 is a diagram illustrating a signal flow for a handover procedure in a conventional BWA communication system.
Referring to FIG. 1, a mobile station (MS) 100 receives a Mobile Neighbor Advertisement (MO_NBR-ADV) message from a serving base station (BS) 120 in step 101 and scans the serving BS 120 and first and second neighbor base stations 140 and 160 by measuring their signal strengths in step 103. After the scanning, the MS 100 transmits a Mobile Mobile Station Handover Request (MOB_MSHO-REQ) message to the serving BS 120 in step 105.
The serving BS 120 transmits Handover notification (HO_notification) messages to the first and second neighbor base stations 140 and 160, notifying that the MS 100 may perform a handover to them in steps 107 and 109. The HO_notification messages include information about the service level of an on-going service for the MS 100.
The first and second neighbor base stations 140 and 160 transmit a response message for the HO_notification messages to the serving. BS 120 in steps 111 and 113. The response messages include information about service levels supported by the neighbor base stations 140 and 160.
In step 115, the serving BS 120 transmits a Mobile Base Station Handover Response (MOB_BSHO-RSP) message including the information about the service levels supported by the neighbor base stations 140 and 160 to the MS 100.
The MS 100 transmits a Mobile Handover Indication (MOB_HO-IND) message to the serving BS 120, notifying that it will perform a handover to the second neighbor BS 160 in step 117. The MS 100 decides on the target base station to which it will perform the handover, taking into account the service levels supported by the neighbor base stations.
The serving BS 120 transmits a Handover confirm (HO_confirm) message indicating the handover of the MS 100 to the second neighbor BS 160 in step 119.
Meanwhile, when the mobile station transmits the MOB_HO-IND message to the serving base station, the serving base station takes time to process the MON_HO-IND message. Even during the processing time, the serving base station transmits a control signal or data to the mobile station.
FIG. 2 illustrates a time delay involved in processing the MOB_HO-IND message in the conventional BWA communication system.
Referring to FIG. 2, a serving BS Modulator-Demodulator (MODEM) 220 receives a Bandwidth Request (BR) code for transmission of the MOB_HO-IND message from a mobile station (MS) 200 in frame n in step 201. The serving BS MODEM 220 processes the BR code and allocates resources for the transmission of the MOB_HO-IND message in frame n+4 in step 203.
The MS 200 transmits the MOB_HO-IND message including information about a target BS 260 to the serving BS MODEM 220 using the allocated resources in step 205.
In step 209, the serving BS MODEM 220 transmits the MOB_HO-IND message to a BS 240 after processing the MOB_HO-IND message. The BS 240 includes a signal processor.
Meanwhile, the MS 200 performs a handover to the target BS 260 after transmitting the MOB_HO-IND message to the serving BS MODEM 220 in step 207.
The serving BS 240 transmits an HO-confirm message to the target BS 260 in step 211.
As described above, a time delay occurs from the time when the MS 200 decodes on the handover and transmits the BR code for transmission of the MOB_HO-IND message to the time when the serving BS MODEM 220 discontinues data transmission.
From the perspective of the mobile station, the mobile station may fail to receive data from the serving base station wholly or partially during the time between the decision on the handover and the transmission of the MOB_HO-IND message.
From the perspective of the serving BS, the serving BS has no idea whether the MS has received data successfully or not during the time between reception of the MOB_HO-IND message and interpretation of the MOB_HO-IND message. Therefore, the serving BS transmits information about data next to data that is considered to have normally been received at the MS to the target BS so that the target BS transmits data to the MS based on the received data information.
To prevent this unnecessary resource consumption and time delay, the MS can notify the target BS of the sequence number (SN) of the last service data unit (SDU) or Automatic Repeat reQuest (ARQ) block successfully received from the serving BS by an SN report header, after network reentry to the target BS.
The target BS transmits an SDU or ARQ block with the SN following the SDU SN included in the SN report header to the MS. Table 1 below illustrates the format of the SN report header.
TABLE 1HT = 1(1)EC = 0(1)Type(3) = 0b110Last(1)SDUSDU SN2SN1(6)MSB(4)SDU SN2 LSB(2)SDU SN3(6)CID MSB(8)CID LSB(8)HCS(8)
In Table 1, numerals in the brackets denote the number of bits in the respective items. SDU SN1 to SDU SN3 are fields indicating the SDU SNs of three connections, when up to three connections are established between the MS and the BS.
The issue is that the current BWA communication system does not label a Medium Access Control (MAC) SDU with an SN. In other words, there is no specified method for indicating an SN of the MAC SDU by an SN report header between the BS and the MS.
The MS may transmit an SN report header including the SN of an ARQ block to the target BS, but the target BS can start data transmission successfully only when the target BS uses the same ARQ scheme and associated variables as used between the MS and the serving BS. Accordingly, there exists a need for a method for allocating an SN to an SDU so that the MS can efficiently start a data transmission/reception procedure with the target BS.